Clean Diesels

What is it?

Diesel is refined from petroleum, like gasoline, but the pollution it produces is harder to control. "Clean diesel" vehicles burn the fuel more efficiently and trap pollutants better. New low-sulfur diesel fuel also pollutes less—much like unleaded gasoline, compared with leaded.

What's good about it?

Diesel contains more energy than gasoline, so cars get about 30 percent better mileage—with greater torque, which drivers feel as low-end acceleration. The extra energy also makes diesels ideal for towing heavy loads, which is why many trucks are diesel-powered.

What's bad about it?

The extra technology in clean diesels can raise the cost by $3,000 or more, compared with gas models. And clean diesels still emit more smog-causing pollutants than the cleanest gas-powered cars. Producing more low-sulfur diesel could also increase greenhouse-gas emissions from refining.

Where would it be most useful?

For large vehicles that generally get poor mileage and trucks used for heavy hauling. A number of states with the toughest emissions requirements—like California, New York, and Massachusetts—effectively ban older diesels, although clean diesels should meet their standards.

How much will it cost?

The price of diesel is usually a bit higher than gasoline, and new low-sulfur diesel is more expensive still. Diesel engines cost more, too. Some drivers might recoup the extra cost through better mileage.

When's it coming?

A couple of clean diesels, like the Mercedes-Benz E320 sedan, are on the market now, with other models from automakers such as Volkswagen, Audi, and Hyundai on the way.

What's taking so long?

Diesels are popular in Europe, where gas is more expensive, but in the United States, pollution problems have relegated them to a small sliver of the car market. If clean diesels pass muster in California—which sets the standards for several other states—the technology could make a comeback.

Who's doing it?

Mercedes, Audi, VW, and other carmakers from Europe—where diesels are most popular—have the most advanced clean-diesel technology. General Motors, Ford, and Chrysler use diesel engines in their pickup trucks. Others plan to introduce clean diesels, too.

Could it be a silver bullet?

No. Since it's a fossil fuel, even clean diesels are not an alternative to hydrocarbons.